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AbstractCladistic analyses of the nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) sequences from 55 samples corresponding to 34 taxa in the genus Armeria reveal that ITS sequence diversity among and within species utterly conflicts with patterns of morphological similarity. Three facts are apparent from the results here reported: (1) different samples of a single subspecies, A. villosa subsp. longiaristata, appear in three of the five major clades; (2) samples of at least one of the six subspecies of A. villosa appear in four of the five major clades; and (3) composition of major clades shows greater congruence with the geographic origin of plants than with the traditional systematic arrangement based primarily on morphology. Specifically, the clades here termed Ia, II, III, and IV each encompass terminals restricted to geographically delimited areas. There are alternative explanations for the ITS pattern, but the most likely one is that nucleotide positions supporting the major clades are due, in some of the samples, to concerted evolution following horizontal transfer (gene flow) rather than to recency of common ancestry. This interpretation is consistent with previous systematic and experimental evidence and implies that reticulation in Armeria may be extensive. Harlan and de Wet (1963, Evolution 17:497-501) proposed the compilospecies concept to account for situations in which a genetically "aggressive" species captures portions of the genome of other sympatric species via extensive introgression. Evidence of extensive reticulation, ecological diversification, and geographic pattern indicates that A. villosa may fit the compilospecies concept, which is here supported on molecular grounds for the first time.